1) Field of the Invention
The present invention relates to a polyimidosiloxane resin, a composition thereof, and a method of applying same. More particularly, the present invention relates to a polyimidosiloxane resin having a high solvent solubility and an excellent heat resistance, and capable of forming shaped articles and coating films with a satisfactory insulating property, a high flexibility and a satisfactory mechanical strength, a polyimidosiloxane composition comprising the polyimidosiloxane resin, and optionally another resinous material, which are dissolved or dispersed in an organic solvent, and capable of forming coating film having a high resistance to curling, a high flexibility and a satisfactory mechanical strength of a shaped article or a flexible circuit board, and a method of applying the polyimidosiloxane composition to form the coating layer on the shaped article or flexible circuit board.
2) Description of the Related Art
It is known that conventional aromatic imide polymers can be used for forming electrical insulating protective film layers on shaped articles. For example, the aromatic imide polymers are utilized to form electrical insulating film layers on solid electric elements, passivation film layers, and interlaminar electrical insulating film layers of semiconductive integrated circuits and of flexible circuit boards.
Generally, the conventional imide polymers have a very poor solubility in organic solvents, and therefore, the protective coating film layers for the electronic elements, for example, a circuit element comprising a copper foil circuit adhered to a flexible substrate through an adhesive layer, are usually formed by coating the electronic element surfaces with a solution of a polyimide precursor consisting of an aromatic polyamic acid in an organic solvent, drying the coated precursor solution layer, and heat-treating the dried precursor layer on the electronic element at a high temperature of from 250.degree. C. to 400.degree. C. for a long time of from 60 to 180 minutes, to imidize the polyamic acid into a corresponding polyimide. This heat treatment at a high temperature for a long time causes a thermal deterioration of the copper foil circuit and the adhesive layer, and the imidization of the polyamic acid causes the polyamic acid to be dehydrated and the resultant cured polyimide layer to be shrunk and thus curled. Further, the dehydration causes a number of fine pores or voids to be formed in the resultant polyimide layer, and thus the porous polyimide layer exhibits a lower insulating property and a reduced protection of the electronic element.
Japanese Examined Patent Publication No. 57-41491 discloses an aromatic imide polymer soluble in an organic solvent. This aromatic imide polymer is, however, disadvantageous in that the bonding (close adhesive) property of the imide polymer to substrate materials, for example, silicon wafers, glass plates and flexible substrate plates, is unsatisfactory, and therefore, the surfaces of the substrate materials to be bonded must be pretreated with a bonding-promotion agent.
To eliminate the above-mentioned disadvantages, U.S. Pat. No. 4,520,075 and Japanese Unexamined Patent Publication No. 58-13,631 disclose a polyimidosiloxane precursor prepared by employing a diaminopolysilicone compound as a diamino component.
This type of polyimidosiloxane precursor, however, is also disadvantageous in that a heat treatment at a high temperature must be carried out to convert the precursor to the corresponding polyimidosiloxane.
Further, U.S. Pat. Nos. 4,586,997, 4,670,497, 4,968,757, and 4,672,099 and Japanese Unexamined Patent Publication Nos. 63-225,629, and 1-121,325 disclose solvent-soluble polyimidosiloxanes. Some of the polyimidosiloxanes have a disadvantage such that the polymers can be produced only by a relatively large number of steps, and thus the production time is very long. Also, some of the polyimidosiloxanes do not include aromatic diamine as a diamine component, and thus have a poor heat resistance. Further, some of the polyimidosiloxanes exhibit a disadvantage such that, when a solution of the polyimidosiloxane is coated on a flexible substrate plate, for example, copper-coated plate, and the resultant solution layer is dried, the substrate plate is significantly curled.
It is also known that an epoxy resin can be utilized to form a protective coating film layer on a surface of an electric circuit element, for example, a flexible circuit element or circuit element for TAB, by applying a solution of the epoxy resin in an organic solvent and drying and curing the resultant epoxy resin solution layer. The epoxy resin protective layer exhibits a satisfactory bonding property to the circuit element surface and a high heat resistance, but is disadvantageous in that, since the cured epoxy resin layer has a high stiffness and a poor flexibility, the circuit element coated with the cured epoxy resin layer is significantly curled, and sometimes a large number of fine cracks are created in the cured epoxy resin layer.